In traditional optical devices, imaging systems often utilize multiple lenses to form high quality images with low aberrations. In addition to the time and expense of fabricating multiple lenses, there is a corresponding increase in system volume and weight. In addition, the increased system volume can lead to problematic layout challenges in many instances. Although multi-lens approaches can be successful for many applications, there are a number of other instances where a multi-lens approach is either sub-optimal or not feasible at all.
Traditional lenses utilize optical materials with a constant index of refraction. The constant index of refraction necessitates that a given lens have a particular size and shape in order to bend (refract) electromagnetic radiation in a desired manner as the electromagnetic radiation passes through the lens. As indicated above, this requirement can represent a significant cost and design issue, particularly for multi-lens optical systems. In addition, the convex or concave shape of traditional lenses can be problematic to incorporate into the architecture of some optical systems. For example, the shape of convex and concave lenses can preclude stable placement of such lenses on a flat surface.
Compared to traditional lenses having a constant index of refraction, lenses fabricated from a material having a gradient index (GRIN) of refraction can provide a number of advantages. Such materials will be referred to herein as “GRIN materials.” Lenses containing a GRIN material can bend electromagnetic radiation differentially depending upon the particular region of the lens through which the electromagnetic radiation travels. Because bending of the electromagnetic radiation is no longer limited by the constant index of refraction of a single material, lenses containing a GRIN material can be fabricated in simpler geometries that can facilitate their disposition in various optical systems. For example, GRIN-based lenses can have a flat surface in certain extreme cases, or can replace a more expensive aspheric lens (e.g., with a simpler spherical lens made of a GRIN material). Moreover, GRIN-based lenses can allow fewer lenses or even one lens to accomplish a similar optical transformation to that provided in comparable multi-lens systems employing traditional lens materials. Hence, GRIN-based lenses can provide significant opportunities for reduction of the size and complexity of various optical systems.
Despite the desirability of GRIN materials, relatively few are known, and they can sometimes be difficult to fabricate. One process for preparing GRIN materials involves ion-exchange modification of a base oxide glass matrix, typically resulting in radial gradient index of refraction in the direction of the ion exchange. Other illustrative processes for producing GRIN materials involve stacking and/or laminating thin layers of various glasses or polymers having differing indices of refraction to form a material having a refractive index that varies throughout the material's thickness, thereby resulting in an axial gradient index of refraction in the stacked elements. Post-production doping of a low-index material with a high-index material, such as through inkjet printing techniques or other liquid-like mixing processes of two or more materials, can also be used to produce a GRIN material.
GRIN materials produced in the foregoing manners and others can have a number of drawbacks. Many of the foregoing fabrication processes are operationally complex, time-consuming, and expensive. When using conventional fabrication processes, it can also be difficult to vary the refractive index satisfactorily across the surface of a lens or in multiple dimensions. Additionally, the magnitude of the refractive index gradient is usually small, and laminated GRIN materials can be subject to delamination under certain environmental or use conditions. From a technological standpoint, conventionally produced GRIN materials can often display a limited transmission window, sometimes due to the spectral properties of the matrix material, particularly throughout the infrared or visible absorption regions of the electromagnetic spectrum. Scattering of electromagnetic radiation can also be problematic in conventionally produced GRIN materials. These factors can limit the breadth of applications where conventional GRIN materials can be satisfactorily used, particularly in systems that transmit multiple wavelengths across specific ranges of electromagnetic radiation, for example. Electromagnetic radiation transparency with limited scatter in a broad wavelength range of 1 to 12 microns can be of particular interest for infrared laser systems, for example. The lack of broadband transparency can limit the applicability of many GRIN materials in various instances.
In view of the foregoing, GRIN materials having a wide transmission window that can be produced by readily available fabrication techniques would be of considerable interest in the art. The present disclosure satisfies the foregoing need and provides related advantages as well.